The present invention relates to nuclear power plants and more particularly to systems for supporting a pressure relief and safety valve system above a pressurizer vessel in a pressurized water nuclear power plant.
A pressurizer vessel is used in a pressurized water nuclear reactor power plant to provide relief for plant coolant overpressure. Usually, such a pressurizer vessel is a vertical, thick walled unit having a hemispherical top or dome, and having its bottom end supported by a flanged skirt.
Relief valves provide a first level of overpressure protection and safety valves provide a second or backup level of overpressure protection. The pressurizer relief and safety valve system includes a manifold or header assembly and piping connections to nozzles in the head of the pressurizer and to discharge outlets. One situation where overpressure protection is needed, for example, is where the entire plant electrical load has been dumped because of electrical operating conditions.
The entire pressurizer valve system must be safely and reliably supported above the pressurizer in accordance with plant safety and performance standards. In earlier plant designs, valve support systems were excessively costly because the valve support system designs were plant dependent.
Subsequently, a basic valve support system-was developed for use as a standardized scheme for supporting pressurizer valves and piping in variously designed pressurized water reactor plants. That basic support system is disclosed in U.S. Pat. No. 4,426,350 entitled VALVE SUPPORT ARRANGEMENT FOR PRESSURIZED IN NUCLEAR POWER PLANT, issued to M. J. Zegar et al. on Jan. 17, 1984 and assigned to the present assignee.
In the Zegar patent, an arrangement for supporting the pressurizer safety and relief valve system is described including a common header supported relative to the side walls of the pressurizer vessel by the use of columnar supports secured to the header and to the pressurizer side wall by the use of lug means. The lug means are attached to the pressurizer side wall and must be capable of supporting the load of the columnar supports and pressurizer safety and relief valve system.
One disadvantage of Zegar support system was that the vessel support lugs had to be large to avoid overstressing from some combinations of load forces under various operating conditions. The vessel wall itself is thus undesirably subjected to heavy load forces under various conditions.
Generally, rigid valve system support is required for dynamic forces such as those stemming from an earthquake. At the same time, flexible support is required to accommodate thermal growth or contraction of the valve system due to changing temperature conditions. The transmittal of large load forces to the vessel lugs in Zegar occurs because all vertical and horizontal load forces and moments about all three reference axes are transmitted to the vessel support lugs.
Continuing development effort accordingly led to subsequent improvements as set forth in U.S. Pat. No. 4,576,788 entitled STRADDLE-TYPE SUPPORT STRUCTURE FOR NUCLEAR POWER PLANT PRESSURIZER VALVES, issued to R. M. Blaushild on Mar. 18, 1986 and assigned to the present assignee and U.S. Pat. No. 4,629,601 entitled STIRRUP-TYPE SUPPORT STRUCTURE FOR NUCLEAR POWER PLANT PRESSURIZER VALVES, issued to R. M. Blaushild on Dec. 16, 1986 and assigned to the present assignee.
In the straddle-type valve support system, the vessel lugs are subjected only to horizontal forces which tend to rotate the lower support ring on which columnar valve supports rest. All other load forces are applied to arcuate sections that interconnect the lower and upper support rings, thereby distributing such forces over the vessel head surface. Heavy loading of the vessel lugs is eliminated, but much more extensive support structure is required and accessibility for inspecting vessel welds is difficult and time consuming.
In the stirrup-type valve support system, some improvement is achieved in lug loading through even distribution of loading forces from the columnar supports to the vessel lugs through a ring girder. The columnar supports are secured to the girder at points spaced from the points at which the girder is secured to the vessel lugs thereby enabling the lug loading forces to be distributed more evenly.
While disassembly of the stirrup-type girder-vessel lug securance is facilitated to enhance vessel head accessibility for weld inspection, overall vessel head access for weld inspections is still difficult and time consuming because of the visual obstruction presented by the relatively complex valve support system structure with the valve support system left in place or, alternatively, because of the overall difficulty of support structure disassembly for more open access to the vessel head.
In all of the prior art pressurizer valve support systems there has been little or no provision for accommodating unit to unit manufacturing differences such as vessel lug hole locations, etc.
There has accordingly continued to be a need for further improvement in the support of a pressurizer safety relief valve system in nuclear power plants. The present invention is directed to achieving significant improvement in pressurizer valve support systems through simpler, more economic structure that provides better vessel head accessibility for weld inspections while meeting valve support safety and performance requirements.